Such that our internal structs can be pretty printed with indented and
type-hinted fields, AND for nested `Struct`-fields call `.pformat()` but
avoiding any recursion errors using `pprint.saferepr()`. Add
a `._sin_props()` iterator over the non-property fields; use it for
`dict` casting when called with `.to_dict(include_non_members=False)`.
Actually, we should also probably figure out how to only pprint like
when required by the user in a REPL or log msg by context-selectively
`pprint.PrettyPrinter` right? Also, if we can generalize decently enough
it'd be cool to maybe patch this in as a util to upstream `msgspec`?
Since it's depended on by `.data` stuff as well as pretty much
everything else, makes more sense to expose it as a top level module
(and maybe eventually as a subpkg as we add to it).
Tyler Goodlet